The term coevolution is used to describe cases where two (or more) species
reciprocally affect each other's evolution. So for example, an evolutionary change in the
morphology of a plant, might affect the morphology of an herbivore
that eats the plant, which in turn might affect the evolution of the plant, which
might affect the evolution of the herbivore...and so on.

Coevolution is likely to happen when different species have close
ecological interactions with one another. These ecological relationships
include:

Plants and insects represent a classic case of coevolution  one
that is often, but not always, mutualistic. Many plants and their pollinators
are so reliant on one another and their relationships are so exclusive
that biologists have good reason to think that the "match" between
the two is the result of a coevolutionary process.

But we can see exclusive "matches" between
plants and insects even when pollination is not involved. Some Central
American Acacia species have hollow thorns and pores at the
bases of their leaves that secrete nectar (see image at right). These hollow thorns are
the exclusive nest-site of some species of ant that drink the nectar.
But the ants are not just taking advantage of the plant  they also
defend their acacia plant against herbivores.

This system is probably
the product of coevolution: the plants would not have evolved hollow
thorns or nectar pores unless their evolution had been affected by
the ants, and the ants would not have evolved herbivore defense behaviors
unless their evolution had been affected by the plants.